In recent years, aluminum nitride (hereinafter also referred to as “AlN”) single crystal is gaining attention as a substrate material for semiconductor devices such as various optical devices and electronic devices, because aluminum nitride single crystal has wide energy band gap, high thermal conductivity and high electrical resistance.
Examples of conventional method of producing an AlN single crystal includes a sublimation method in which an AlN crystal material is placed in a crucible and then the sublimated AlN is grown as single crystal (e.g. see JP H10-53495 A (PTL 1)). In the sublimation method, a powder of a single crystal material, and a powder of an oxide which reacts with the material to decompose and vaporize AlN when heated, are mixed to obtain a mixed powder, and the mixed powder is heated in a nitrogen atmosphere, or in a nitrogen atmosphere containing hydrogen or carbon, at a temperature lower than the sublimation temperature or the melting temperature of the material, to decompose and vaporize the mixed powder into AlN, so that the decomposed and vaporized component grows as crystal on a substrate.
However, the conventional method of producing an AlN single crystal by sublimation is likely to cause cracks in crystal during the growing process of the desired single crystal. When cracks occur in a crystal, it is extremely difficult, or may be impossible in some cases, to use a wafer made of such a crystal as a reliable substrate for device production. That is to say, commercial devices used for epitaxial growth, photo lithography and other device processes require a circular wafer of uniform thickness and perfect shape. Cracks impair the utility of the wafer even when they are too small to cause separation of the wafer. Therefore, solution to the problem of cracks during the AlN crystal growth is extremely important for further development of nitride-based electronic equipment.
The sublimation method includes, for example, heteroepitaxial growth using a heterogeneous single crystal as the substrate, and homoepitaxial growth using a homogeneous single crystal as the substrate. The substrate can be fixed to a pedestal (the upper part of the crucible) using an adhesive, which is a known method of fixing the substrate (see JP 2002-60297 A (PTL 2)). In this case, a difference in coefficient of thermal expansion between the substrate and the pedestal may cause thermal stress. The homoepitaxial growth using a homogeneous substrate as the substrate can reduce the thermal stress, yet cannot solve the problem of high possibility of crack occurrence in the produced single crystal.
A known means to prevent the cracks caused by a difference in coefficient of thermal expansion is to, as described in SEI Technical Review, No. 168-(103), published in March, 2006 (NPL 1), increase the thickness of the crystal to reduce the curvature radius caused by thermal stress.
However, the technique of thickening the crystal to reduce the curvature radius is based on the assumption that there is a difference in coefficient of thermal expansion only between the crystal and the substrate, with no reference to the difference in coefficient of thermal expansion between the substrate and the pedestal. Taking the difference in coefficient of thermal expansion between the substrate and the pedestal into consideration, it is insufficient to merely thicken the single crystal when using the sublimation method to produce a single crystal.
Therefore, JP 2013-159511 A (PTL 3) proposes a single crystal production apparatus with which a grown single crystal such as a grown MN single crystal can be taken out, for example, with no cracks or breaks.